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L
OA c
L
Ar
Ar B(OH)
2
Pd
Pd
L
OAc
L
OA c
L
Ar
L
L
Ar
Pd
Pd
L
AcO
-
OA c
L
L
L
Ar
Ar
Pd
P
d
OA c
L
loose stereocontrol
tight stereocontrol
Scheme 9.147 Bidentate ligands do not guarantee bidentate chelation.
monodentate fashion, which is a likely source of less effective stereo-
differentiation (Scheme 9.147).
The other probable source is random wandering of the double bond,
leading to partial racemization, which may take place because in oxidative
Heck reactions PdH intermediates are not rapidly scavenged by base and are
more long-lived, bringing havoc into the orderly course of the enantiose-
lective pathway.
Therefore, more efforts are required to develop enantioselective oxidative
Heck reactions, both by ligand design to seek ligands that would not be
dechelated during the reaction and the use of nucleofugic counterions to
permit the polar pathway with a stable bidentate chiral ancillary and a free
coordination location for catalytic action. It should be taken into consider-
ation that there are more problems in oxidative pathways to be dealt with: (a)
in the case of phosphine ancillaries, oxidative conditions lead to oxidation of
phosphine sites and thus to disruption of bidentate binding and deterior-
ation of stereocontrol; and (b) the oxidant itself often provides non-labile
ligands for palladium (e.g. Cu acetate gives acetate, O
2
gives OH, OOH or -O-
ligands, quinones may give semiquinone or phenolate ligands, etc.), which
would block the polar pathway, hence a judicious choice of terminal oxidant
is essential if tighter stereocontrol is desired.
In a later study, Jung and co-workers obtained very important clues on
which direction the ligand design for enantioselective deborylative reactions
can take.
407
A tridentate CNO ligand based on a heterocyclic carbene
modified with a chiral amino alcohol was used as a preformed complex to
deliver high optical yields although at the expense of reaction rate and
material yields. The idea is indeed highly motivating - if one cannot reliably
control the anionic ligand in the coordination shell, such can be provided in
advance as the third arm of a tridentate ligand. The third site should be
chosen to be able to behave only as a hemi-labile ligand capable of binding
the olefin. Such hemi-lability is likely to be enhanced by binding borate
during transmetallation (Scheme 9.148).
This work also provides an instructive example demonstrating that it is
the bidentate ligand and not the polar (cationic) state of the Pd centre which
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